Process for leaching rare earth elements

ABSTRACT

Described herein is a process for stepwise leaching of all rare earth elements capable of forming peroxide and superoxide compounds, in particular cerium, lanthanum, neodymium, europium, from minerals containing these elements, namely from bastnaesites, orthites, chevkinites, and britholites.

CROSS-REFERENCE TO RELATED APPLICATION

This U.S. nonprovisional application claims priority to German Application No. 102019003556.9 filed May 19, 2019, the entire contents of which are incorporated herein by reference.

TECHNICAL DESCRIPTION OF THE INVENTION

The present invention relates to the field of mining industry and specifically to the leaching of rare earth elements (REE).

The direct leaching of rare earth elements takes place by means of hydrochloric acid, sulphuric acid, nitric acid or mixtures thereof.

In patent RU 2547369 C2 dated 18 Feb. 2013 “Process for the complex treatment of residues from Domanik formations”, leaching is carried out by a multi-stage treatment with sulphuric acid of varying concentration, inter alia with the maximum aggressive concentration of 15 to 25%, at temperatures from 65° C. to 160° C. and at pressures from atmospheric pressure to 13 bar.

The disadvantage of this process is that the use of aggressive sulphuric acid concentrations requires the use of equipment made of highly alloyed molybdenum steel. Moreover, when sulphuric acid is used, the fluorides decompose at temperatures above 95° C. to form gaseous and water-soluble compounds, which impair the environmental situation.

The patent WO 2012/149642 A1 (publication date 8 Nov. 2012) or RU 2013153535 of 3 May 2012 “Process for extraction of rare earth elements from different ores” proposes to carry out leaching with hydrochloric acid, sulphuric acid, nitric acid or mixtures thereof. It is also recommended to use the most aggressive concentrations of acids, temperatures from 85 to 175° C. and elevated pressures.

The disadvantages of the proposed process are also the need to use expensive equipment made of highly alloyed steel and a significant negative impact on the environment.

In the patent description of WO 2016/164600 A1 “Leaching aid for metal extraction” (international publication of 13 Oct. 2016), which mainly focusses on the extraction of gold, silver, platinum, palladium, titanium, nickel and copper, it is indicated that other non-ferrous metals and rare earth elements can also be leached using this process. According to the process, crushed raw materials are pretreated with various synthetic surfactants—cationic, anionic, amphoteric and non-ionic—at a consumption of up to 10 kg per ton of raw material.

After this treatment, a heap leaching or an agitation leaching is suggested. The duration of the process can be several years for the heap leaching process and 6 to 12 days for the agitation leaching process. The leaching is carried out at a ratio of solid:liquid=1:1 to 1:3. For agitation leaching, a temperature of up to 200° C. and a pressure of 1 to 51.7 bar can be used. The following substances are used for leaching:

-   -   Acids: HNO₃, HF, HCl, H₂SO₄, H₃PO₄, HClO₄;     -   Bases: Na₂CO₃, NaOH, NH₃;     -   Complexing agents: NaCN, KCN, Ca(CN)₂, thiosulfates, thiourea,         thiosulfuric acid, dithiooxamides, substituted dithiooxamides;     -   Oxidizing agent—ozone.

FeSO₄, FeCl₃, FeCl₂ and other halogen-containing compounds are reacted as auxiliary materials.

The disadvantages of this process are:

-   -   Use of synthetic surfactants. Surfactants primarily reduce the         surface tension of water, with a decreasing dissolution of         oxygen in the water. The majority of synthetic surfactants is         poorly decomposed or not at all decomposed by the         microbiological flora of the purification plants and this result         in swamp formation in the bodies of water into which they are         introduced. In addition, many surfactants can form stable         compounds with heavy metals (the elimination thereof requires         temperatures up to 100° C., lasts up to 8 hours and is carried         out using expensive materials suitable for the decomposition of         these compounds), which also strongly suppress aerobic         microflora, leading to swamp formation.     -   A very long process duration—with agitation leaching from 6 to         12 days and with heap leaching up to several years.     -   The recommended temperature of up to 200° C. and pressures of up         to 51.7 bar require the use of expensive equipment—autoclaves,         which must be made of highly alloyed steels with additional         Teflon protection due to the use of highly corrosive acids and         bases.     -   The complexing agents proposed for metal leaching are highly         toxic and require very complex systems for the disposal of the         residual amount in waste water and specific solid waste storage.     -   Use of ozone. The solubility of ozone in water at a pressure of         1 bar is about 10 mg per 1 litre. In order to achieve a         technological effect, higher concentrations are required, which         are achieved under high pressure. In this case, there is a risk         of formation of explosive ozonides, which can destroy not only         the equipment but the entire production.     -   Iron sulphates and chlorides are used as auxiliary substances in         connection with acids in order to shift the equilibrium in acid         leaching of the required metals, namely by forming insoluble         compounds with the acid residues of the mineral raw materials.         The closest and already known pre-publication to the proposed         technical solution is described in the patent EP 0265547 A1 of         30 Oct. 86 “Verfahren zur Gewinnung von Seltenen Erden und         gegebenenfalls Uran und Thorium aus Schwermineralphosphaten”         (prototype of the present invention). In this process, the         starting raw material, namely phosphorites, more precisely         monazites, is comminuted to a size of 37 microns or 400 mesh in         a planetary mill with simultaneous activation. Take 20 g of         monazite, add 65 g of 40% nitric acid and a stoichiometric         amount of iron salt to cause an exchange reaction with         phosphates of rare earth elements. This mixture is placed in an         autoclave made of high-alloy steel with Teflon protection. In         the autoclave, the pressure is increased to 7-9 bar and the         temperature to 170-190° C. The process is carried out for 20-40         minutes. The mixture is then cooled and filtered.

In this regard, the following is to be noted. When REE is leached with acids, the chemical equilibrium applies, whereby the degree of leaching of the REE depends on the type of acid and its concentration. As a rule, the degree of leaching rarely reaches 70% when highly concentrated acids are used. According to the principle of Le Chatelier, it is necessary to convert the reaction products into non-reactive substances (to be removed from the reaction zone by the extraction of water-insoluble substances or with a gas) or into other substances that would not react with the mineral residue in order to shift the equilibrium of the chemical process to the right.

In order to increase the efficiency of nitric acid, a mechanical activation of the mineral up to a particle size ensuring better accessibility of the valuable components to nitric acid was carried out in the process according to the above patent. When nitric acid acts on the phosphorite of a rare earth element, rare earth nitrate and free phosphoric acid are formed. A water-soluble iron salt is used so that the phosphoric acid does not react with the REE nitrate and no equilibrium occurs. Iron reacts with the phosphoric acid to form insoluble iron phosphate. In this way the degree of leaching of REE increases, which are converted from insoluble phosphates into soluble nitrates. At high pressure peroxides and ozonides are not formed.

The disadvantages of this process are:

-   -   the process is proposed only for phosphorites of the REE (REE         may have different acid residues in the ore, see below);     -   aggressive nitric acid is used;     -   the process requires high temperature and high pressure;     -   expensive equipment made of corrosion-resistant materials is         required;     -   the mechanical activation of the raw material requires the use         of planetary mills, which have very complex kinematics which are         sensitive to imbalance. At present, such mills are only         developed for laboratory purposes and the production of         respective industrial plants cannot be controlled. Further,         since the comminution of the starting raw material is carried         out to a particle size which is smaller than is usual in         industry (in industry the particles are reduced to 63-75 microns         and in the process of the prototype to up to 37 microns), the         implementation of this process is basically only possible in the         laboratory.     -   the process is not ecologically clean: in part the nitric acid         will decompose into nitrogen oxides that pollute the atmosphere         and in part the nitric acid will remain in the solid residue         (filter cake) and thus contaminate the soil and the groundwater.

In the implementation of the claimed invention, wastes were used as raw materials, which result from the extraction of rare earth elements from the mineral raw materials of the Sichuan deposit (China) according to a standard process and had a content of 12% REE.

The raw materials, which are subjected to leaching in accordance with the present process, include the following components:

1. calcite

2. baryte-BaSO₄— one of the main components

3. fluorite-CaF₂— one of the main components

4. bastnaesite-Ce[CO₃]F

5. aegirine-Fe,Mg,Ca SiO₂

6. orthite-(Ca,Ce,Y)(AlFe)₃Si₃O₁₂(O,OH) and Ce,Ca,Mg,Al₂[SiO₄][Si₂O₇]O(OH)

7. chevkinite-Fe₂La₂Ce₂Ti₃O₈[Si₂O₇]

8. britholite-Ca₂(Ce,Y)₃OH[SiO₄]₃

The raw materials presented can be processed with conventional acid thermal processes, however thereby fluorine is released into the gaseous medium or—if nitric acid is used—the nitrogen oxides, i.e. substances which have a negative influence on the environment and on the conditions of industrial leaching.

The aim of the present invention is to develop an environmentally friendly and cost-effective process for the industrial leaching of rare earth elements, which makes it possible to dispense with the use of expensive equipment made of corrosion-resistant materials and is feasible on a broad basis of raw materials of minerals containing REE. The basis for the development of the new process was the fact that the REE to be leached were in a bi-trivalent state and it was necessary to choose the conditions for the chemical exchange with an available hydrolysable salt whose metal could be in a bi-trivalent state. The most easily accessible salt for this process is iron trichloride, which is hydrolysable to hydrochloric acid and is an oxidizing agent that can be easily reduced from a trivalent to a divalent state. At the same time, iron trichloride can be used, which is produced in the course of chlorine reuse in the production of NaOH from NaCI, which makes the cost of iron trichloride with 100% concentration about 2 times lower than the cost of hydrochloric acid with 40% concentration. When working with iron trichloride, it is also sufficient to use equipment based on widely used chromium steels having a chromium content of 13 to 17%.

The above factors of cost reduction make the claimed process much more cost-effective than all the processes described above. In the present process, there is no activation comminution step of the raw materials, since the raw materials used were already subjected to a comminution to a particle size of 75 microns when they were processed in the conventional way before leaching according to the process.

The process claimed here comprises the following steps:

1. Preparing the reaction mixture consisting of the leaching agent (preferably of a hydrolysable iron salt, particularly preferably of iron trichloride), the solvent (preferably water) and the starting raw materials by mixing them at the ratio of solid:liquid=1:5 and a pH between 1 and 3.

2. Increasing the temperature of this mixture to 80-100° C. while stirring.

3. Keeping the mixture at this temperature for a specified time (at least about 1 hour) while stirring.

4. Filtration of the resulting suspension, i.e. separation of the liquid phase from the solid phase, for example on a Buchner funnel or on any other filter. The REE leached out in this process step will pass into the liquid phase.

5. Multiple washing of the solid residue carried out as a repetition of process steps 1. to 4., wherein in this case as starting raw materials the solid residue is used which is obtained either at the end of the fourth process step or at the end of the preceding washing cycle after the fifth process step. The number of repetitions of the washing stage is determined by the required degree of leaching of REE.

6. The final stage of the process, aimed at extracting REE from the solution and recovering from the solid concentrate in pure form, is carried out once and consists of the following steps:

-   -   a. Processing the obtained solution with air until the divalent         iron contained in the solution is completely converted into         trivalent iron.     -   b. Treating the solution obtained with sodium hydroxide at room         temperature until a pH between 9 and 12 is reached. Thereby         sodium salts of peroxide and ozonide compounds of rare earth         elements and iron(III) salts are precipitated from the solution,         the latter precipitating in the form of iron(III) oxide         hydrates.     -   c. The solution is then filtered and the sediment is first dried         for about 4 hours at 90-100° C., whereby the peroxide and         ozonide compounds of the REE decompose and transform into REE         oxide hydrates, while iron (Ill) is still present in the dried         sediment.     -   d. The dried sediment is then treated at room temperature with         an amount of oxalic acid, which is equimolar to the trivalent         iron contained therein, whereby only the trivalent iron         dissolves to form oxalates and the REE oxide hydrates remain in         the solid sediment.     -   e. The final filtration of the solution is performed to separate         solid REE concentrate in pure form from a iron(III) oxalate         solution. At the end of this process step, we obtain the rare         earth elements in pure form ready for further use.

The final stage of the process has been specially developed as an alternative technological operation to the standard method of precipitation of REE from the solution and its purification from the admixtures, because the process according to the invention does not allow the final recovery of solid REE concentrate from the solution in pure form in a standard way. At the same time, an important technical and economic advantage of the final stage of the process described herein is that the purification of REE from the iron(III) admixtures does not require a 20-fold but only an equimolar amount of oxalic acid in relation to the iron.

The present process in the general embodiment described above represents the proposed solution to the technical problem posed and is within patent claim 1 (independent claim). Patent claim 2 (dependent claim) focusses on a specific embodiment of the present process in which the minerals to be leached out are components of a stockpile material consisting of residues of a previous leaching. The advantage of the present process, associated with the substantial saving of oxalic acid in the final stage, is present in patent claim 4 (dependent claim).

The final leaching degree of REE according to the claimed process can be up to 100%, which is not characteristic for acid-thermal leaching processes, which result in a lower leaching degree of REE.

Furthermore, the invention preferably concerns a process for stepwise leaching of rare earth elements from minerals containing these elements, namely from bastnaesites, orthites, chevkinites, britholites, which have previously been comminuted to at least the particle size of 75 micrometres, comprising the following steps:

-   -   an initial stage comprising the following steps: a first step of         preparing the reaction mixture consisting of a leaching agent, a         solvent and mineral raw materials containing rare earth         elements; a second step of leaching REE by heating the obtained         reaction mixture to the working temperature of 80-100° C. and         maintaining it at the given temperature with simultaneous         stirring and a pH between 1 and 3; and a third step of         subsequently filtering the resulting suspension to form the         liquid solution of REE compounds and a solid residue;     -   one or more intermediate stages;     -   a final stage comprising the following steps: a first step of         treating the solution obtained as a result of the previous         process steps with air until the divalent iron contained in the         solution is completely transformed into trivalent iron; a second         step of treating the solution obtained in the previous step with         sodium hydroxide at room temperature until a pH between 9 and 12         is reached; a third step of filtering the solution obtained in         the previous step and drying the sediment obtained thereby at         90-100° C. for about 4 hours; a fourth step of processing the         sediment dried in the previous step with oxalic acid at room         temperature, thereby forming a solution; a fifth step of final         filtration of the solution obtained in the previous step,         thereby forming a solid REE concentrate in pure form, wherein         iron trichloride is used as leaching agent and water is used as         solvent, and wherein the ratio solid:liquid in the reaction         mixture is 1:5, the duration of holding the reaction mixture at         the working temperature is at least about 1 hour and all         intermediate stages of the process are repetitions of the         initial stage thereof, the total number of intermediate stages         typically being 2 to 3, the solid residue formed as a result of         each preceding process stage being used as a starting raw         material in each subsequent intermediate stage, wherein a         reduction-oxidation process takes place parallel to the         hydrolysis of iron trichloride, which results in the formation         of water-soluble peroxide and/or ozonide compounds of the rare         earth elements and iron(II) chloride, so that both the iron and         the REE compounds remain in the solution and do not enter the         sediment.

The preferred embodiments described above and in the pending claims apply analogously to this process.

Example 1: The leaching process for rare earth elements was carried out as follows. Take 1 kg of crushed raw materials and put them in a container, pour 5 dm³ of tap water (solid:liquid=1:5) over them and add 200, 150, 100, 50 or 30 g FeCl₃. The mixture was thoroughly mixed, then heated to 80-100° C. with simultaneous stirring and kept at this temperature for 1 hour, also with simultaneous stirring. The suspension was then filtered, dried and the solid residue weighed. The results of the mass loss experiments are shown in Table 1.

TABLE 1 Iron trichloride consumption, (in % to the initial mass of the raw materials) 20 15 10 5 3 Mass loss of raw materials, 5 7 11 15 7 (in % to their initial mass)

As can be seen from the table, the maximum mass loss is observed when the consumption of iron trichloride is 5% in relation to the initial mass of the raw materials studied. This can be explained by the fact that the degree of hydrolysis of iron trichloride increases with decreasing concentration and that the hydrochloric acid released during hydrolysis interacts intensively with the components of the mineral raw materials. It should be noted that under these conditions practically all of Fe⁺³ is converted to Fe⁺² and the rare earth elements are converted to the water-soluble chloride, peroxide and ozonide compounds. This is shown by the fact that upon further processing of the resulting solution with oxalic acid, oxalates of the divalent iron precipitate, while the oxalate of the trivalent iron is a water-soluble compound. All ozonide compounds of rare earth elements are sublimable when heated, this effect was observed when the water soluble products were carefully evaporated and dried.

Already in the 19th century it was proven that all rare earth elements can form peroxide and superoxide compounds during their chemical transformations. Obviously, this could also affect the optimal leaching conditions. A further reduction in iron trichloride consumption could reduce these opportunities for leaching REE.

The process carried out with a 5% consumption of iron trichloride in relation to the initial mass of the raw materials investigated represents a preferred embodiment of the present process, which is present in patent claim 3 (dependent claim).

Example 2: Process as in example 1, except that only 50 g iron trichloride was added. Thereafter, the reaction mixture was thoroughly mixed, then heated to 80-100° C. with simultaneous stirring and kept at this temperature for 1 hour, also with simultaneous stirring, then filtered, dried, weighed and analysed. The same sequence of process steps was then repeated twice with the solid residue obtained after the previous repetition of this sequence. In total, all the above-mentioned process steps were carried out three times, with a corresponding decrease in the mass of the starting raw materials or the solid residue, respectively, after each repetition. The results of the analyses are shown in Table 2.

TABLE 2 number of the respective processing results of the analyses of the initial raw mass of solid residue degree of leaching of material sor solid after the respective REE after the respective residue processing, g processing, %. 1 850 50 2 750 78 3 700 90

With respect to the given data, the leaching of REE with iron trichloride is analogous to the extraction processes. Under the experimental conditions, the absolute value of fluorine in the original sample and after the treatments remained unchanged.

The solution obtained is then processed by treatment with air to convert the divalent iron into the trivalent state. Next, the solution is treated with NaOH up to a pH of 9 to 12, thereby precipitating rare earth elements and iron. The solution is filtered and the sediment is first dried for about 4 hours at 90-100° C. and then treated with an equimolar amount of oxalic acid in relation to the iron. The solution is filtered and a concentrate of rare earth elements is obtained in the sediment.

The following chemical processes take place in the application of the present process:

FeCl₃+H₂O=FeCl₂OH+H⁺, the pH is 2.3 at room temperature, at T=80-100° C. -1.0.

Fe⁺³=Fe⁺² −e

H⁺ +e=H

2H+O₂=H₂O₂

Me≡(O—X)₃+3H₂O₂+3H⁺=Me≡(O—O—H)₃+3H₂O+3X

Thus, water-soluble ozonide compounds of rare earth elements are formed. During the treatment of the resulting solution with the oxygen contained in the air, oxidation from divalent iron to trivalent iron takes place:

Fe⁺² −e=Fe⁺³

The oxalates of divalent iron are insoluble in water and those of trivalent iron are soluble in water.

When processed with sodium hydroxide, sodium salts of the ozonide compounds of rare earth elements and iron(III) hydroxide precipitate. During heating (drying) the ozonide compounds are decomposed and during treatment with oxalic acid only the iron(III) oxalate enters in the solution. According to standard methods, a 20-fold excess of oxalic acid is required for the extraction of rare earth elements and according to the proposed method an equimolar amount thereof relative to iron is sufficient.

Compared to known methods, the claimed invention has the following significant differences:

-   -   The leaching of rare earth elements does not require the use of         aggressive acids;     -   The leaching of rare earth elements requires the use of         iron(III) salts.

The following causal relationship exists between the distinguishing features of the leaching of rare earth elements with the iron trichloride and the technical problem to be solved

-   -   The optimum concentration of iron trichloride in relation to the         initial mass of the mineral raw materials is 5%.     -   When rare earth elements are leached out with iron trichloride,         the chemical equilibrium sets in.     -   In order to obtain a high degree of leaching, a repeated         treatment should be carried out.     -   During leaching, chloride, peroxide and ozonide compounds of         rare earth elements are formed.     -   During leaching with iron trichloride, the trivalent iron is         converted into a divalent compound.     -   To obtain the REE concentrate, the solution is first treated         with air, then with sodium hydroxide, the sediment is then         separated, dried and treated with an equimolar amount of oxalic         acid.

During the investigation of the given and related technical fields, no other technical solution was found to have the features that distinguish the present process from the prototype. Accordingly, the criteria of novelty and inventive step are fulfilled in this invention. 

1. A process for stepwise leaching of all rare earth elements capable of forming peroxide and superoxide compounds, in particular cerium, lanthanum, neodymium, europium, from minerals containing these elements, namely from bastnaesites, orthites, chevkinites, britholites, which have previously been comminuted to at least the particle size of 75 micrometers, comprising the following steps: an initial stage comprising the following steps: a first step of preparing the reaction mixture consisting of a leaching agent, a solvent and mineral raw materials containing rare earth elements; a second step of leaching REE by heating the obtained reaction mixture to a working temperature of 80-100° C. and maintaining it at the given temperature with simultaneous stirring and a pH between 1 and 3; and a third step of subsequently filtering the suspension thus obtained to form the liquid solution of REE compounds and a solid residue; one or more intermediate stages; a final stage comprising the following steps: a first step of treating the solution obtained as a result of the previous process steps with air until the divalent iron contained in the solution is completely transformed into trivalent iron; a second step of treating the solution obtained in the previous step with sodium hydroxide at room temperature until a pH between 9 and 12 is reached; a third step of filtering the solution obtained in the previous step and drying the sediment obtained thereby at 90-100° C. for about 4 hours; a fourth step of processing the sediment dried in the previous step with oxalic acid at room temperature, thereby forming a solution; a fifth step of final filtration of the solution obtained in the previous step, thereby forming a solid REE concentrate in pure form, wherein iron trichloride is used as leaching agent and water is used as solvent, and wherein the ratio solid:liquid in the reaction mixture is 1:5, the duration of holding the reaction mixture at the working temperature is at least about 1 hour and all intermediate stages of the process are repetitions of its initial stage, the total number of intermediate stages typically being 2 to 3, the solid residue formed as a result of each respective preceding process stage being used as a starting raw material in each respective subsequent intermediate stage, wherein a reduction-oxidation process takes place parallel to the hydrolysis of iron trichloride, which results in the formation of water-soluble peroxide and/or ozonide compounds of the rare earth elements and iron(II) chloride, so that both the iron and the REE compounds remain in the solution and do not enter in the sediment.
 2. The process according to claim 1, wherein the minerals used are constituents of a stockpile material from residues of a previous leaching.
 3. The process according to claim 1, wherein the consumption of leaching agent during carrying out the initial stage and each intermediate stage is constant and is from 3 to 20%, preferably 5%, relative to the initial mass of the mineral raw materials.
 4. The process according to claim 1, wherein the amount of oxalic acid used in the fourth step of the final stage is equimolar relative to the trivalent iron contained in the dried sediment treated with the oxalic acid.
 5. The process according to claim 2, wherein the consumption of leaching agent during carrying out the initial stage and each intermediate stage is constant and is from 3 to 20%, preferably 5%, relative to the initial mass of the mineral raw materials.
 6. The process according to claim 2, wherein the amount of oxalic acid used in the fourth step of the final stage is equimolar relative to the trivalent iron contained in the dried sediment treated with the oxalic acid.
 7. The process according to claim 3, wherein the amount of oxalic acid used in the fourth step of the final stage is equimolar relative to the trivalent iron contained in the dried sediment treated with the oxalic acid. 